Data segregation in a storage device

ABSTRACT

An example method includes providing at least two data storage areas in a memory, providing a first amount of over-provisioning for a first of the at least two data storage areas and a second amount of over-provisioning for a second of the at least two data storage areas, categorizing data based on a characteristic of the data, and storing the data in one of the at least two data storage areas based on the categorization.

BACKGROUND

Computers and other electronic devices generally require a medium inwhich digital data can be stored and retrieved. Data storage devicescome in a variety of forms and serve a variety of purposes. Thesedevices can be typically broken down into two general categories: solidstate and non-solid state storage devices.

Non-solid state devices are devices that contain moving parts. Sometypical non-solid state storage devices are hard disk drives, compactdisc read/write drives and disks, digital video disc read and read/writedrives and disks, floppy disks, tape drives and probe memory devices.These storage devices move one or more media surfaces and/or theassociated data head relative to one another to position the data headrelative to a desired location or area on the media. The data is thenwritten to or read from this data location. In disk drives, for example,data is stored on a disk that rotates at an essentially constantvelocity. By moving the head over the rotating disk, all memorylocations or sectors of the disk can be accessed.

Solid state storage devices differ from non-solid state devices in thatthey typically have no moving parts. Solid state storage devices may beused for primary storage of data for a computing device, such as apersonal computer, workstation computer, or server computer. An exampleof a solid state storage device is flash memory.

Flash memory, e.g., NAND flash memory, is comprised of a number ofcells, with each cell being similar to a metal-oxide semiconductor (MOS)field-effect transistor (FET) and having a gate, a drain, and a source.In addition, the cell includes a “floating gate.” When a voltage isapplied between the gate and the source, the voltage difference betweenthe gate and the source creates an electric field, thereby allowingelectrons to flow between the drain and the source in the conductivechannel created by the electric field. When strong enough, the electricfield may force electrons flowing in the channel onto the floating gatevia Fowler-Nordheim quantum tunneling. The number of electrons on thefloating gate determines a threshold voltage level of the cell.

Flash memory may typically be broken into two categories: single-levelcell (SLC) and multi-level cell (MLC). In SLC flash memory, two voltagelevels are used for each cell, thus allowing SLC flash memory to storeone bit of information per cell. In MLC flash memory, more than twovoltage levels are used for each cell, thus allowing MLC flash memory tostore more than one bit per cell.

Flash memory may suffer from more wear than non-solid state devices.That is, charges may be applied and removed, e.g., the cell may bewritten and erased, a finite number of times before the structure of thecell may become physically compromised. Although MLC flash memory iscapable of storing more bits than SLC flash memory, MLC flash memorytypically suffers from more wear than SLC flash memory.

SUMMARY

In one example, the disclosure is directed to a method that comprisesproviding at least two data storage areas in a non-volatile memory,providing a first amount of over-provisioning for a first of the atleast two data storage areas and a second amount of over-provisioningfor a second of the at least two data storage areas, categorizing databased on a characteristic of the data, and storing the data in eitherthe first or the second of the at least two data storage areas based onthe categorization.

In another example, the disclosure is directed to a system thatcomprises a non-volatile memory and at least one controller configuredto provide at least two data storage areas in the non-volatile memory,provide a first amount of over-provisioning for a first of the at leasttwo data storage areas and a second amount of over-provisioning for asecond of the at least two data storage areas, categorize data based ona characteristic of the data, and store the data in either the first orthe second of the at least two data storage areas based on thecategorization.

In another example, the disclosure is directed to a computer-readablemedium comprising instructions encoded on the computer-readable mediumthat, upon execution, cause a controller within a device to provide atleast two data storage areas in a non-volatile memory, provide a firstamount of over-provisioning for a first of the at least two data storageareas and a second amount of over-provisioning for a second of the atleast two data storage areas, categorize data based on a characteristicof the data, and store the data in either the first or the second of theat least two data storage areas based on the categorization.

These and various other features and advantages will be apparent from areading of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example solid state drive(SSD).

FIG. 2 is a block diagram of an example data temperature detectionmechanism.

FIG. 3 is a block diagram depicting an example temperature detectiontechnique that utilizes implied hotness.

FIG. 4 is a conceptual diagram illustrating data that is not groupedaccording to temperature.

FIG. 5 is a conceptual diagram illustrating data that is groupedaccording to temperature, in accordance with certain techniques of thisdisclosure.

FIG. 6 is a conceptual diagram illustrating write amplification whendata is not grouped according to temperature.

FIG. 7 is a conceptual diagram illustrating write amplification whendata is grouped according to temperature, in accordance with certaintechniques of this disclosure.

FIG. 8 is a conceptual diagram depicting example garbage collectionunits that include embedded sequence counters for temperature detectiontechniques in accordance with this disclosure.

FIG. 9 is a block diagram depicting an example temperature detectiontechnique that utilizes sequence numbers along with implied hotness todetermine data temperature.

FIG. 10 is a block diagram depicting another example temperaturedetection technique that utilizes sequence numbers along with impliedhotness to determine data temperature.

FIG. 11 is a block diagram illustrating an example system including theSSD of FIG. 1.

FIG. 12 is a block diagram illustrating the example SSD of FIG. 1configured in accordance with various techniques of this disclosure.

FIGS. 13 and 14 are example graphs depicting the probability densityfunction of future host writes.

FIG. 15 is an example graph depicting the remaining wear capability ofone hundred erasure blocks, sorted by wear rather than by erasure blocknumber.

FIG. 16 is a flow diagram illustrating an example method for segregatingdata in a non-volatile memory, in accordance with various techniques ofthis disclosure.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for partitioning asolid state drive into two or more data storage areas, providingover-provisioning for each of the data storage areas, and storing datain the data storage areas based on characteristics of the data. As willbe described in more detail below, over-provisioning generally refers tothe difference between the amount of data made available to the host (ora user thereof) and the actual amount of data that can be stored onsolid state drive. That is, a solid state drive may be capable ofstoring a greater amount of data than advertised to a user and/or user'shost. In certain examples, such over-provisioning and segregation ofdata may be used for reducing wear in a solid state drive (SSD).

FIG. 1 is a block diagram illustrating an example solid state drive(SSD) 10. In the example of FIG. 1, SSD 10 includes controller 12,volatile memory 14, and non-volatile memory 20. Volatile memory 14 maycorrespond to random access memory (RAM). Non-volatile memory 20corresponds to long-term storage of SSD 10. In general, SSD 10 includesone or more flash dies, such as flash die 22, each of which include aplurality of erasure blocks 24A-24N (“erasure blocks 24”). Flash die 22may generally be one of a number of dies included within a semiconductorchip, and an SSD 10 may comprise one or more semiconductor chips. Eachof erasure blocks 24 includes a plurality of physical blocks.

In the example of FIG. 1, erasure block 24A includes physical blocks26A-26N (“physical blocks 26”) and erasure block 24N includes physicalblocks 28A-28N (“physical blocks 28”). Physical blocks may also bereferred to throughout this disclosure as “pages.” It should beunderstood that the letter N in the reference numerals above is anon-negative integer and that the use of N with respect to differentreference numbers should not be understood as implying that the numberof items are equal. For example, the number of physical blocks 26 is notnecessarily equal to the number of physical blocks 28. Likewise, thenumber of erasure blocks 24 is not necessarily equal to the number ofphysical blocks 26 or the number of physical blocks 28.

In the example of FIG. 1, SSD 10 includes controller 12. Controller 12comprises hardware, such as one or more processors, microprocessors,digital signal processor (DSPs), application specific integratedcircuits (ASICs), field programmable gate arrays (FPGAs), or any otherequivalent integrated or discrete logic circuitry for performing thefunctions ascribed to controller 12. In other examples, a computingdevice (not shown) coupled to SSD 10 may implement the functionalityascribed to controller 12. For example, an SSD may not includecontroller 12, and instead a software driver implemented by an operatingsystem of the computing device may perform the functions of controller12. In this manner, a processor or other control unit separate from SSD10 may control SSD 10. In some examples, controller 12 may include amemory (not shown). A memory in controller 12 may be used to storemetadata. For example, controller 12 may include memory that storesmetadata that represents a frequency of write access of data written toSSD 10 (e.g., temperature of the data).

In general, SSD 10 stores data for a long-term period in a physicalblock, such as physical block 26A of erasure block 24A. Host data isreferenced by logical block addresses (LBA) of host blocks. Controller12 also writes data regarding each physical block to a header in thephysical block. The header generally includes metadata that describesdata of the corresponding physical block. The header may include datasuch as a number of LBAs stored in the corresponding physical block anda size or length of the header.

Before writing data to a physical block, such as physical block 26A,controller 12 writes the data to volatile memory 14. Controller 12accumulates a full physical block of data in volatile memory 14 beforewriting the physical block to non-volatile memory 20, e.g., in physicalblock 26A. As controller 12 receives LBAs to be stored from a host(e.g., a computing device coupled to SSD 10), controller 12 writes thereceived LBAs to volatile memory 14. In some examples, controller 12also compresses the data in volatile memory 14.

Compressing data results in effective over-provisioning. In general,over-provisioning refers to the difference between the amount of datamade available to the host (or a user thereof) and the actual amount ofdata that can be stored on SSD 10. That is, SSD 10 may, in someexamples, be capable of storing a greater amount of data than advertisedto a user and/or user's host. For instance, SSD 10 may advertise thecapability to store X bytes of data to the host, but in actuality may becapable of storing Y bytes of data where Y>X. When SSD 10 can store morethan X bytes of data, SSD 10 is said to be over-provisioned. The value Xrepresents an advertised amount of total storage in SSD 10. SSD 10 mayuse compression techniques, additional physical memory, or a combinationthereof to provide over-provisioning. For example, if (X=Y) andcontroller 12 can compress data by a ratio of 2:1, then ½ X is notnecessary to store data.

There are several advantages that stem from using over-provisioning inan SSD. For example, SSD 10 may operate faster if there isover-provisioning because when controller 12 is searching for anavailable physical block, e.g., page, to write new data, there may bemore physical blocks available to be written to at the time controller12 is searching. With sufficient over-provisioning, controller 12 mayavoid the need to copy data internally in order to get an erasure blockfully available for writing. Thus, with sufficient over-provisioning,controller 12 may quickly locate an available physical block for writingdata, when needed, because there are fewer garbage collecting operationsneeded.

It should be noted that an erasure block is completely erased in orderfor any physical block in the erasure block to be written to. Theselection of erasure blocks from which to collect valid data and theinternal copying of data from those erasure blocks to free up entireerasure blocks for writing is referred to as “garbage collection.” Ifgarbage collection is using fewer resources, then more back-end flashresource space may be available for host data to be written to, therebypotentially increasing performance.

Another advantage of over-provisioning is reduction in wear to thenon-volatile memory, e.g., SLC flash. Copying data internally viagarbage collection techniques causes additional wear to the flash. Thewear caused by garbage collection is not the result of the hostproviding data that needs to be written, but rather the internaldata-copying techniques necessary for effective data storage innon-volatile memory 20.

For example, assume that data may be randomly written to erasure blocksin the non-volatile memory, that a mapping strategy allows any LBA to beput in any portion of the non-volatile memory, and that metadata (e.g.,a table) keeps track of the location of the LBAs in physical space. Asdata in erasure blocks 24 are re-written into different ones of erasureblocks 24, holes containing invalid data are “created” in the erasureblocks. That is, erasure blocks 24 contain valid data as well as data ofLBAs that are no longer valid. Until the entire erasure block is erased,no data may be written to the erasure block. Garbage collectiontechniques copy all the valid data from a first erasure block to asecond erasure block, e.g., a partially filled erasure block, therebyfreeing up the first erasure block to be erased. With sufficientover-provisioning, the frequency for performing garbage collection maybe reduced because, with random writes, it is more likely controller 12will find an erasure block that is available for receiving new data,i.e., an erasure block with little or no data that needs to be copiedout of the block before erasure to make the block available once againfor writing. As such, over-provisioning provides a wear-benefit.

In an SSD, data is often grouped together for mapping efficiency reasonsand/or flash architecture reasons. If the host changes any of the datain the SSD, such grouping may require that the entire group of data bemoved and mapped to another region of the storage media. In the case ofan SSD utilizing NAND flash, this grouping affects all data within anerasure block, whether the fundamental mapping unit is an erasure block,or a programming page within an erasure block. All data within anerasure block is affected because when an erasure block is needed tohold new writes, any data in the erasure block that is still “valid”(i.e., data that has not been superseded by further data from the host)is copied to a newly-mapped unit so that the entire erasure block can bemade “invalid” and eligible for erasure and reuse. If all the valid datain an erasure block that is being copied share one or morecharacteristics (e.g., temperature, as described in more detail below),there may be significant performance and/or wear gains from keeping thisdata segregated from data with dissimilar characteristics.

For instance, grouping data based on the data's “temperature” may beadvantageous. The temperature of data refers to a frequency of read orwrite access of the data. Data that has a higher frequency of writeaccess is said to have a higher temperature (or be “hotter”) than datathat has a lower frequency of write access. Data may categorized, forexample, as “hot” and “cold”, “hot,” “warm,” and “cold,” or the like,based on predetermined or configurable threshold levels. Or, rather thancategorizing data as “hot,” “warm,” and “cold,” a numerical scale may beused (e.g., 1-10 or 1-100). When data needs to be written to media,either via garbage collection, host writes, or any other operation, thetemperature of the data may be determined and data with similartemperatures may be grouped together using techniques of thisdisclosure. Depending on the workloads and observed or characterizedphenomena, the system may designate any number ‘N’ hotness groups (e.g.,if N=2, then data may be characterized as hot or cold and if N=3, thendata may be characterized as hot, warm, or cold, and so forth). Withineach grouping of temperature, the system may order the data so that asdata becomes hotter or colder, the system is able to determine whichlogical data space will be added or dropped from a group, as will bedescribed in more detail below.

As mentioned above, the higher the temperature of data, the more likelyit is that the data will be rewritten again soon. By grouping data withsimilar temperatures, it is more likely that the data will be rewrittenat a similar frequency. As such, most of the data within a garbagecollection unit (a region of data storage that is a target of a garbagecollection process, e.g., one or more erasure blocks) will be stale(e.g., superseded by newer host data) and the write amplification causedby garbage collection may significantly decrease. Write amplification(also referred to as “wear amplification” throughout this disclosure)refers to the amount of media data written in order to satisfy theamount of host data written. For example, in order to write 1 megabyteof host data, 0.5 megabytes may also need to be rewritten via internalgarbage collection techniques in order to free up a sufficient number oferasure blocks to write the 1 megabyte of host data. Lower writeamplification may extend the life and increase the performance of thedevice.

FIG. 2 is a block diagram of an example data temperature detectionmechanism. In FIG. 2, the temperature of host write data 200 and thetemperature of valid data collected via garbage collection operation 202(which causes write amplification) may be compared using temperaturecomparator 204 and, based on the comparison, placed in a logical blockaddress (LBA) of “hot” group 206, “warm” group 208, or “cold” group 210.Temperature comparator 204 may take into account all metrics andcalculations that characterize data as hot, cold, or any value inbetween. Temperature comparator 204 may characterize data with numeroustemperature gradients, and within a given range there may be a finergrain sorting of temperature.

One possible implementation of temperature detection utilizes impliedhotness. Hotness may be implied from the fact that data being writtenfrom the host is likely to be “hot,” otherwise the host would not bewriting the data. Conversely, garbage collected data is likely to be“cold,” otherwise the host would have previously re-written the data.

FIG. 3 is a block diagram depicting an example temperature detectiontechnique that utilizes implied hotness. All newly written host data200, which is assumed to be hot based on implied hotness, is placedwithin “hot” group 206. In other words, the data being written by thehost may be categorized as having a first designation, i.e., “hot.” Allvalid data collected via garbage collection operation 202, which isassumed to be cold based on implied hotness, is placed within “cold”group 210. In other words, the data being collected via a garbagecollection operation may be categorized as having a second designation,i.e., “cold.” Numerous other categorizations are envisioned other thansimply two categorizations. Although the technique shown in FIG. 3 maybe expanded to incorporate other metrics for grouping data based ontemperature, FIG. 3 illustrates one straightforward approach forsegregating data based on the temperature of data.

FIG. 4 is a conceptual diagram illustrating data that is not groupedaccording to temperature. In FIG. 4, five garbage collection units (GCU0 through GCU 4) are depicted in seven states, as shown by columns220-226, in their order in time. In initial drive state 220, GCU 0through GCU 2 are populated with valid data and GCU 3 through GCU 4 areempty. In state 221, new host data is written to the device and the newhost data, e.g., hot data, fills the formerly empty locations within GCU3. The newly written host data causes data within several locationswithin GCU 0 through GCU 2 to become stale. Locations within GCU 4remain empty in state 221.

In state 222, garbage collection occurs and the GCU with the most numberof stale pages (e.g., GCU 0 in state 221), is garbage collected. Thegarbage collected data is assumed to be cold, and the cold data iswritten to GCU 4. In state 223, new host data is written, filling up theremaining empty locations within GCU 4 and causing data in GCU 2 and GCU3 to become stale. GCU 4 now includes both cold (garbage collected) dataand hot (newly written) data. The intermixing of cold and hot dataoccurs after each garbage collection operation.

In state 224, garbage collection occurs and the GCU with the most numberof stale pages (e.g., GCU 2 in state 223), is garbage collected. Thegarbage collected data is assumed to be cold, and the cold data iswritten to empty GCU 0. In state 225, new host data is written, fillingup the remaining empty locations within GCU 0. The newly written hostdata causes data in GCU 1, GCU 2, and GCU 3 to become stale. State 226depicts the drive once steady-state is reached. In state 226, i.e.,steady state, GCU 0, GCU 1, GCU 2, and GCU 4 include both hot and colddata. The mixing of data temperatures may cause write amplification anddecrease the endurance and performance of the drive.

FIG. 5 is a conceptual diagram illustrating data that is groupedaccording to temperature, in accordance with certain techniques of thisdisclosure. In FIG. 5, five garbage collection units (GCU 0 through GCU4) are depicted in seven states, as shown by columns 230-236, in theirorder in time. States 230-232 in FIG. 5 are similar to states 220-222 inFIG. 4 and, as such, will not be described in detail again. In state 233of FIG. 5, new host data is written to a previously designated “hot”GCU, e.g., GCU 0. This is in contrast to state 223 of FIG. 4 where host(i.e., hot) data is written to a GCU that includes cold data, e.g., GCU4 of FIG. 4.

In state 234 of FIG. 5, garbage collection occurs and the GCU with themost number of stale pages (e.g., GCU 2 in state 233), is garbagecollected. The garbage collected data is assumed to be cold, and thecold data is written to a previously designated “cold” GCU, e.g., GCU 4.In state 235, new host data is written, filling up the remaining emptylocations within a designated “hot” GCU, e.g., GCU 0, and causing datain GCU 1 and GCU 3 to become stale. In this manner, cold (garbagecollected) data is segregated from hot (newly written) data. Thesegregation of hot and cold data will continue throughout the life ofthe device.

State 236 depicts the drive once steady-state is reached. In state 236,i.e., steady state, data is segregated based on temperature. Forexample, GCU 2 and GCU 4 contain only cold data, while GCU 0 and GCU 1contain either hot data or stale data. The next garbage collectionoperation (not shown) would again select the GCU with the most number ofstale pages. As shown in FIG. 5, GCU 0 has the most number of stalepages, namely five, at the time of garbage collection. This iscontrasted against FIG. 4, where GCU 0, GCU 1, and GCU 4 each have twostale pages at the time of garbage collection. Thus, using thetechniques of this disclosure, write amplification is reduced by afactor of 2.5 (five pages compared to 2 pages). Consistently achieving areduction in write amplification of 2.5× may translate into about a 2.5×increase in lifetime and write performance.

FIG. 6 is a conceptual diagram illustrating write amplification whendata is not grouped according to temperature. In FIG. 6, two garbagecollection units (GCU 0 and GCU 1) are depicted in four states, as shownby columns 240-243, in their order in time. State 240 depicts an initialdrive state in which GCU 0 and GCU 1 have equal amounts of hot and colddata, with intermixing of data temperatures. State 241 depicts thesteady state condition of the drive, illustrating intermixing of datatemperatures.

In state 242, the hot data is reprogrammed, i.e., rewritten by the host,resulting in equal amounts of cold data and stale data in GCU 0 and GCU1. In state 243, garbage collection occurs and the GCU with the mostnumber of stale pages is garbage collected. As seen in state 242, thereis an equal amount of stale data and valid data in GCU 0, thus writeamplification is increased by four data groups (i.e., four locations ofdata in GCU 0 must be copied to another location within GCU 0). State243 depicts the four cold data groups rewritten to GCU 0 via the garbagecollection techniques.

FIG. 7 is a conceptual diagram illustrating write amplification whendata is grouped according to temperature, in accordance with certaintechniques of this disclosure. In FIG. 7, two garbage collection units(GCU 0 and GCU 1) are depicted in four states, as shown by columns250-253, in their order in time. State 250 depicts an initial drivestate in which GCU 0 and GCU 1 have equal amounts of hot and cold data,with intermixing of data temperatures. State 251 depicts the steadystate condition of the drive, illustrating segregation of datatemperatures, in particular, cold data in GCU 0 and hot data in GCU 1.

In state 252, when the hot data is reprogrammed, i.e., rewritten by thehost, all of the data within GCU 1 becomes stale. In state 253, garbagecollection occurs and the GCU with the most number of stale pages isgarbage collected. However, in this case, garbage collection is “free.”That is, because there is no valid, e.g., cold, data remaining withinGCU 1 that needs to be copied elsewhere (due to the fact that data wassegregated by temperature), there is no increase in write amplificationduring garbage collection. In contrast to FIG. 6 where there is a halfGCU empty (GCU 0), FIG. 7 depicts a full GCU empty (GCU 1) available fornew writes, or rewrites via garbage collection, achieved using thetechniques of this disclosure.

FIG. 8 is a conceptual diagram depicting example garbage collectionunits that include embedded sequence counters for temperature detectiontechniques in accordance with this disclosure. Although implied hotnessmay provide a reasonably accurate estimation of the likelihood of databeing rewritten, implied hotness may not be sufficient on its own todetect temperature. For example, using only implied hotness fortemperature detection, if all the “hot” data units within the same GCUare rewritten, then some “hot” data in other GCUs will be miscategorizedas “cold” data.

To prevent this miscategorization, a sequence number (e.g., timestamp ormonotonically increasing number based on the number of host writes) maybe used to predict a better relative temperature for data beingrecycled, as shown in FIG. 8. When data is being written to a GCU, thecurrent sequence number may be appended to the data. When data isrecycled, the sequence number associated with the data may be used bythe hotness detector to pool items that are being recycled with otheritems of a similar sequence number. If the recycled sequence number iswithin a certain proximity, e.g., a predetermined threshold value orrange, to the current sequence number being assigned to the data beingwritten by the host, then the data being recycled may be consideredwarm, as shown in FIG. 9 below. However, if the recycled sequence numberis not within a certain proximity, e.g., a predetermined threshold valueor range, of the current sequence number, then the data being recycledmay be considered cold data. In this manner, a sequence number mayprovide better granularity of temperature for data being recycled with aminimal cost.

FIG. 8 depicts three example garbage collection units, namely GCU 260,GCU 262, and GCU 264, each GCU having N pages and header 266. Eachheader 266 includes a 32-bit sequence number. The cost of including a32-bit sequence number per GCU of size 256 kibibytes (KiB) would beabout 0.001% of the overall storage of the GCU. With larger GCUs, thepercentage of the counter relative to the size of the GCU woulddecrease.

FIG. 9 is a block diagram depicting an example temperature detectiontechnique that utilizes sequence numbers along with implied hotness todetermine data temperature. All data includes a sequence number, e.g., atimestamp, as shown and described above with respect to FIG. 8. In FIG.9, a current sequence number 201 is appended to host write data 200. Allnewly written host data 200, assumed to be hot based on implied hotness,is placed within “hot” group 206. Current sequence number 201, alongwith all valid data collected via garbage collection operation 202, areinput into temperature comparator 204. Temperature comparator 204compares current sequence number 201 to the sequence number of the validdata recycled via garbage collection operation 202 and determines adifference value between the sequence numbers being compared. If thedifference value between the sequence number of the valid, garbagecollected data and current sequence number 201 is within a predeterminedrange (e.g., set by a user or a default setting), then the valid databeing recycled may be considered warm and placed within “warm” group208. However, if the difference value between the sequence number of thevalid, garbage collected data and current sequence number 201 is outsideof the predetermined range, then the data being recycled may beconsidered cold data and placed within “cold” group 210. Thus, thevalid, garbage collected data recycled via a garbage collectionoperation is categorized as having a second designation, e.g., “warm” or“cold,” based on the determined difference value. In this manner, asequence counter may provide better granularity of temperature for databeing recycled via garbage collection operations. In some exampleimplementations, the sequence numbers of the data may be updated afterbeing categorized as having a second designation such as “warm” and“cold.”

In another example implementation, new host write data may becategorized into one of two or more designations. In such an example,temperature comparator 204 may compare current sequence number 201assigned to the host write data 200 to a sequence number previouslyassigned to previously written host data. The previously written hostdata is superseded by newly written host data 200. As such, thepreviously written host data is no longer valid data and will not begarbage collected. Temperature comparator 204 may determine a differencevalue between current sequence number 201 and the sequence numberpreviously assigned to the previously written host data. Based on thedifference value, the data being written by the host may be categorizedas having a first designation.

For instance, if the difference value between the current sequencenumber 201 and the sequence number previously assigned to the previouslywritten host data falls within a first predetermined range, then thenewly written data may be categorized as “hot,” for example. The newlywritten data may be categorized as “hot” because the first predeterminedrange indicates a predetermined closeness in time between writes. If thedifference value between the current sequence number 201 and thesequence number previously assigned to the previously written host datafalls outside of the first predetermined range, then the newly writtendata may be categorized as “warm,” for example, indicating that there ismore time between writes. In other examples, rather than just having afirst predetermined range, there may be a second predetermined range, athird predetermined range, and so forth, in order to provide for morefiner granularity in categorizing the heat of the host write data. Itshould be noted that these techniques of categorizing the newly writtenhost data may be combined with the techniques for categorizing the validdata being collected via a garbage collection operation.

FIG. 10 is a block diagram depicting another example temperaturedetection technique that utilizes sequence numbers along with impliedhotness to determine data temperature. In FIG. 10, a hotness indicator,e.g. small counter, is stored with each LBA range. The LBA range may be,for example, a single LBA, groups of LBAs, e.g., 8 LBAs in a logicalpage, or even larger groups. The hotness indicator may be small toconserve space, e.g., 4 bits per group.

There may be dynamic ranges, based on counter values, which indicatewhether the data is “hot,” warm, or “cold.” It should be noted that thedynamic ranges may be adjusted to provide for increased granularity ofmore than three temperatures. By way of specific example, with a 4-bitcounter, the “hot” group 206 may start with any sequence number greaterthan 12, but then may decrease to greater than 10 if there are notenough elements in the temperature pool. The sequence number may beincremented at increment block 211 on every host write 200, as there issome level of hotness if the host is writing data. When data are garbagecollected at garbage collection operation 202, the difference inprevious sequence counter 212 and current sequence counter 201 maydetermine scaled decrement 213 for the hotness counter.

If the previous and current sequence numbers are within a predeterminedrange, then the hotness counter may not decrement at all, e.g., whenextremely hot data is packed with other hot data and the GCU is recycledvery quickly. If the previous and current sequence numbers are outsidethe predetermined range, but still close together, the counter maydecrement by one. If the previous and current sequence numbers areoutside the predetermined range and not close together, then the scaleddecrement may take away more two or more counts from the hotnesscounter. The scaled decrement may create a hotness indicator that tracksthe hotness of the recent activity and that limits the memory of thesystem, which is beneficial to tracking hotness. Tracking the hotness ofrecent activity and limiting the memory of the system is beneficialbecause hotness may be more useful within a certain time period thanover the entire lifetime of the product.

The techniques described above detail several methods that may beimplemented in hardware, software, firmware, or any combination thereof,in order to determine and compare the temperature of data. In accordancewith further techniques described in this disclosure and as will bedescribed in more detail below, an SSD may be separated into two or moredata storage areas, with each of the two or more data storage areasbeing provided an amount of over-provisioning. Data to be written by ahost and/or data to be copied internally within the SSD via garbagecollection techniques may be categorized based on one or morecharacteristics of the data, and then the data may be stored in one ofthe two or more data storage areas of the SSD based on thecategorization. For example, data may be characterized based on afrequency of write access of the data. Data that has a higher frequencyof write access may be stored in a first data storage area, and datathat has a lower frequency of write access may be stored in a seconddata storage area. In some examples, the first data storage area forstoring data having a higher frequency of write access may be providedwith more over-provisioning than the second data storage area forstoring data having a lower frequency of write access.

Throughout the remainder of this disclosure, the frequency of writeaccess of data will be referred to in terms of the characteristic oftemperature, e.g., heat. Data that is written or updated frequently isconsidered “hotter” than data that is written once or updatedinfrequently. Conversely, data that is written once or updatedinfrequently is “colder” than data that is written or updatedfrequently. As such, data categorized based on temperaturecharacteristics may, at the most basic level, be categorized as “hot”data or “cold” data. Of course, the data need not be simplycharacterized as “hot” or “cold.” Rather, numerous degrees of heatbeyond simply “hot” or “cold” may be applied to the data to furtherrefine the characterizations. For example, there may be three levels ofheat, ten levels of heat, one hundred levels of heat, etc.

FIG. 11 is a block diagram illustrating an example system including theSSD of FIG. 1. In FIG. 11, system 30 is shown including a host 40 andsolid state drive 10. Host 40 may comprise a wide variety of devices,such as one or more of a personal digital assistant (PDA), aworkstation, a personal computer, a laptop computer, a mobile telephone,a media player, or any other device capable of transmitting data to orreceiving data from a storage media. Host 40 may communicate with solidstate drive 10 via a host interface (not depicted). Solid state drive 10further includes controller 12, non-volatile memory 20, e.g., NAND flashmemory, memory 21, and analog-to-digital converter(ADC)/digital-to-analog converter (DAC) 50. Non-volatile memory 20 iscomprised of a plurality of cells 52 with each cell 52 configured tostore electric charge corresponding to a value comprising a certainnumber of bits.

Controller 12 may be configured to perform a number of tasks, includingexecuting instructions 54 stored in memory 21 that allow controller 12to read data from and write data to non-volatile memory 20, via ADC/DAC50. Controller 12 comprises hardware, such as one or more processors,microprocessors, digital signal processor (DSPs), application specificintegrated circuits (ASICs), field programmable gate arrays (FPGAs), orany other equivalent integrated or discrete logic circuitry forperforming the functions ascribed to controller 12.

For example, when host 40 transmits digital signals comprisinginformation to be stored in solid state memory 10 and, in particular, incells 52 of non-volatile memory 20, controller 12 controls ADC/DAC 50 toconvert the digital signal to analog signals, e.g., voltage levels.ADC/DAC 50 may have a resolution that is greater than, e.g., a greaternumber of quantization levels, a resolution of the particular cells(e.g., a number of possible bit values that can be represented by theparticular cells) of the data flash. For example, if non-volatile memory20 is a 3-bit MLC flash, the ADC/DAC 50 may have more than 2³, or eight,quantized levels, to represent one of the three-bit combinations, e.g.,codewords, of “111,” “110,” “101,” “100,” “011,” “010,” “001,” or “000.”ADC/DAC 50 may divide the three-bit combinations into most-significantbits (MSBs) and least-significant bits (LSBs) and program the MSBs andLSBs separately. For example, ADC/DAC 50 may convert the MSBs of athree-bit combination to a quantized voltage level. Then, controller 12applies the quantized voltage level to one of cells 52, thereby storingelectric charges representing the MSBs of the three-bit combination. Inparticular, controller 12 causes electric charges representing the bitcombination to be stored on the floating gate of a cell of non-volatilememory 20. After programming the MSBs, ADC/DAC 50 may perform a similaroperation for the LSBs of the bit combinations. In this manner,controller 12 may progressively program bits from coarse voltage tofiner voltage over time, for example, in different programming steps.

Similarly, when host 40 transmits a request for information previouslystored in solid state drive 10 and, in particular, in cells 52 ofnon-volatile memory 20, controller 12 controls ADC/DAC 50 to convert theparticular voltage level representing the bit combination stored on thefloating gate of a cell of non-volatile memory 20 to a digital signal.ADC/DAC 50 quantizes an estimate of the amount of charge on the floatinggate of the cell to a particular voltage level that represents a bitcombination. Controller 12 then transmits the bit combination to host40. In this manner, host 40 may read data from and write data to solidstate device 10.

Using the techniques of this disclosure, controller 12 may estimate the“temperature” or “heat” of the data being rewritten by host 40 as wellas the heat of the data being copied internally via garbage collectiontechniques. As indicated above, segregating data based on one or morecharacteristics of the data, e.g., the “temperature” of the data, mayprovide wear gains over the life of the SSD. Controller 12 may estimatethe “heat” of data being rewritten by host 40 any number of ways. Forexample, controller 12 may assume the maximum possible heat of thisdata. That is, controller 12 may set an internal bookkeeping heatestimate for data written by the host to a maximum value.

In another example, controller 12 may calculate an estimate of the heatof data by a mixture of the estimated current heat, which is inverselyproportional to how long it has been since the data was last updated,and the previously estimated heat. In such an example, the heat may becalculated as follows:

$\begin{matrix}{{T_{k}\lbrack i\rbrack} = \left( {\frac{\alpha}{T_{k - 1}\lbrack i\rbrack} + {\left( {1 - \alpha} \right) \cdot \left( {{t_{k}\lbrack i\rbrack} - {t_{k - 1}\lbrack i\rbrack}} \right)}} \right)^{- 1}} & (1)\end{matrix}$where T is the temperature, or heat, of the data for map unit [i], where(0≦α<1) represents the exponential “memory” of past temperature (e.g., aweighting factor in favor of the previous heat estimate instead of thetime since the last write), and where t_(k) is the time that the hostdata of interest was written the k^(th) time.

In another example, controller 12 may calculate the heat of data by amixture of maximum heat (assumed so, since host 40 just rewrote thedata) and the heat estimate from the prior time the host wrote the data.In such an example, the heat may be calculated as follows:T _(k) [i]α ^((t) ^(k) ^([i]−t) ^(k-1) ^([i])) ·T _(k-1) [i]+(1−α)·T_(max)  (2)In equation (2) above, the exponential term (t_(k)[i]−t_(k-1) [i]) isused to cause the previous estimate to “cool” based on the amount oftime that elapsed since the previous estimate.

Controller 12 may also estimate the “heat” of data that is about to becopied from at least a first garbage collection unit at least a secondgarbage collection unit via garbage collection techniques (in contrastto data being rewritten by the host, as described above). A garbagecollection unit is a region of data storage that is a target of agarbage collection process (e.g., one or more erasure blocks, one ormore tracks of a magnetic disk, one or more die, one or more mappableunits, and the like). Controller 12 may estimate the heat of data thatis about to be copied via garbage collection techniques in any number ofways, including, but not limited to the following:

(a) 1/(time since the data of interest was last mapped, i.e., written tothe erasure block that is being garbage-collected);

(b) 1/(time since the erasure block containing a mapped unit waserased);

(c) 1/(time since the erasure block containing the data of interestbegan to be filled); and

(d) The fraction of the erasure block that has already been invalidatedby host writing, multiplied by (b) or (c) above. It should be noted thatbeing garbage-collected might indicate minimal heat.

“Time,” as used above, can be calculated in a number of ways and used todetermine the heat of data. “Time” may be calculated, for example, asfollows: power-on time (time that the SSD is aware of, e.g., when theelectronics were powered up); real-time, which includes powered-offtime; the number of mapping blocks written by the host; and the numberof mapping blocks written or allocated inside the SSD.

Using the techniques described above, the temperature of data beingrewritten by the host and the temperature of data that is about to becopied via garbage collection technique may be estimated. If controller12 of SSD 10 maps pages within erasure blocks, and not erasure blocks,non-constant wear amplification may occur. Wear amplification caused bythe ongoing remapping of pages of data is bounded as follows for uniformand independently-selected mappable locations:

$\begin{matrix}{{WA} < {1 + \frac{1}{Overprovisioning}}} & (3)\end{matrix}$If SSD 10 is divided into subsets, the wear amplification, WA, of eachsubset depends on the over-provisioning of that subset of the SSD.

FIG. 12 is a block diagram illustrating the example SSD of FIG. 1configured in accordance with various techniques of this disclosure.Using various techniques of this disclosure, controller 12 provides SSD10, in particular non-volatile memory 20, with two or more subsets, ordata storage area. FIG. 12 depicts two data storage areas, namely firstdata storage area 60 and second data storage area 62. Controller 12provides a first amount of over-provisioning to first data storage area60 and a second amount of over-provisioning to second data storage area62. In particular, controller 12 provides a first amount ofover-provisioning to first data storage area 60 by providing a number oferasure blocks 24 to first data storage area 60 and advertising to ahost, for example, a smaller number of erasure blocks, the difference inbytes between the advertised and provided amounts being the amount ofover-provisioning in first data storage area 60. Similarly, controller12 provides a second amount of over-provisioning to second data storagearea 62 by providing a number of erasure blocks 24 to second datastorage area 62 and advertising to the host a smaller number of erasureblocks, the difference in bytes between the advertised and providedamounts being the amount of over-provisioning in the second data storagearea 62.

For example, in FIG. 12, controller 12 has provided four erasure blocks,namely erasure blocks 24A-24D, to first data storage area 60, andcontroller 12 has provided twenty two erasure blocks, namely erasureblocks 24E-24Z, to second data storage area 62. Assume that both firstdata storage area 60 and second data storage area 62 advertise to a hostthe capability to store the amount of bytes contained in two erasureblocks. Hence, second data storage area 62 has more over-provisioningthan first data storage area 60 because second data storage area 62 hasa higher ratio of unadvertised-to-total erasure blocks (20/22) than thatof first data storage area 60 (2/4). Of course, the number of erasureblocks shown in first and second data storage areas 60, 62 in FIG. 12 isonly for the purposes of illustration. First and second data storageareas 60, 62 may have a much larger number of erasure blocks providedfor over-provisioning.

Controller 12 categorizes data based on a characteristic of the data,e.g., temperature, and then stores the data in one of the data storageareas based on the categorization. For example, in FIG. 12, becausesecond data storage area 62 has more over-provisioning than first datastorage area 60, controller 12 may store hotter data in second datastorage area 62 to take advantage of the reduced wear amplification thatresults from the increased over-provisioning of second data storage area62. Controller 12 may store colder data in first data storage area 60.

After identifying the temperature of data and then segregating the dataof different temperatures into different data storage areas, e.g.,erasure blocks, advantageous garbage collection techniques may then beutilized in order to provide over-provisioning for the different storageareas to improve wear amplification. In a typical garbage collectiontechnique, the controller, e.g., controller 12, selects erasure blockswith higher hole-density (i.e., having lower valid data density) forgarbage collection. If, using the techniques of this disclosure,controller 12 instead garbage collects cold data into well-packed colderasure blocks even when the erasure blocks are observed to have a lowerhole-density (i.e., still having higher valid-data density) than at thepoint at which hot data will be garbage-collected, some erasure blockswill be made available for rewrites that are unlikely to be needed forrewrites of this cold data in the near future.

By “prematurely” garbage collecting cold data (i.e. garbage-collectingcold data with a lower hole-density than hot data that is eligible forgarbage collection), controller 12 makes available to the system moreover-provisioning for hot data. The increased over-provisioning reducesthe wear amplification caused by the hot data, thereby allowing laterperformance improvement in the portion of the SSD that must copy dataduring garbage collection. Controller 12 may use one or more factors todetermine the erasure blocks from which to garbage collect. Examplefactors include, but are not limited to, the following: the amount ofwear of the erasure block, e.g., wear as indicated by the remainingerasure counts; the amount of invalid data, e.g., data that has alreadybeen mapped to newer locations; and the temperature of the data.Regarding the temperature of the data, in some examples, the data in anerasure block may have different temperatures, but the heat of all datain a page may be defined to be the same. That is, controller 12 mayestimate heat on every logically-mappable unit, or aggregate heatestimates. Depending on the implementation, an aggregated region may bean EB, a page, or may be the smallest-possible separately-mapped unit,the logical mapping unit (whether it is compressed or not).

According to the techniques of this disclosure, controller 12 may selecta set of erasure blocks from which to garbage collect that minimizeswear amplification. These techniques may improve performance becausethey minimize the amount of data to be copied in order togarbage-collect an erasure block. For the purposes of the descriptionbelow, it is assumed that there is a set of available-to-write-intoerasure blocks at all times (or, at a minimum, negligibly-populatederasure blocks), and that the garbage collection process is responsiblefor keeping this set populated with a non-zero number of erasure blocksso that a data allocator can place newly-written host data into anavailable location.

Controller 12 may analyze an estimated 2-D probability density function,f(T, D), of garbage-collectable erasure blocks and determine the valuesof (T, D) that, when garbage-collected, minimize the total wear inducedduring ongoing operation, including this garbage collection. Theindependent variables T, D of f(T,D) are as follows: T is the estimatedtemperature of data (approximately the frequency of rewrite of anaverage valid page in an erasure block at (T,D), equal to 1/time untilthe average valid page rewrite); and D (“deadness”) is the portion of anerasure block that no longer contains valid data (e.g., at the moment anerasure block is finished filing with completely valid data, the erasureblock has a deadness of zero).

The total amount of wear on erasure blocks may be defined as follows:

$\begin{matrix}{{TotalWear} = {\sum\limits_{T}{{{WearAmplification}(T)} \cdot {{WriteFrequency}(T)}}}} & (4)\end{matrix}$

In order to minimize TotalWear, a different deadness threshold D isselected for each T, and WearAmplification(T) is back-computed for agiven level of over-provisioning. The variable WearAmplification(T) ofequation (4) is equal to NPPEB/D(T). The appropriate value of D(T) foreach binned value of temperature T (indexed by i) may be found bysolving equation (5) below for D[i] using analytical orcomputational/iterative methods:

$\begin{matrix}{\frac{{D\lbrack i\rbrack} - {N\; P\; P\; E\; B}}{N\; P\; P\; E\; B} = {\mathbb{e}}^{\frac{{{EB}{\lbrack i\rbrack}} \cdot {D{\lbrack i\rbrack}}}{K{\lbrack i\rbrack}}}} & (5)\end{matrix}$where D[i] is the approximate level of “deadness” that will be selectedfor garbage collection for a particular temperature, T, NPPEB is thenumber of pages per erasure block, EB[i] is the number of erasure blocksfor a particular temperature T[i], and K[i] is the number of mappedpages of host data at temperature T[i]. It should be noted that, due toupdates to heat estimates, some of the mapped pages K[i] of host datamight be moved into other pages K. This movement means either that theover-provisioning in the i^(th) temperature may be incrementallyaffected (e.g., over-provisioning may be reduced in the i^(th)temperature, thereby increasing wear amplification caused by hotterdata), or that more erasure blocks may need to be allocated to the setof erasure blocks EB in the i^(th) temperature in order to allow thesame deadness threshold D to be used without affecting over-provisioningin the i^(th) temperature.

Stated another way, controller 12 may divide temperature T into bins,the bins represented by indices [i]. Then, controller 12 may adjustEB[i], the number of erasure blocks for a particular temperature T[i],in order to adapt to changing K[i], the number of mapped pages of hostdata at temperature T[i]. In this manner, controller 12 minimizes thetotal amount of wear on erasure blocks, TotalWear, in equation (4). Insome examples, controller 12 may modify deadness threshold D[i] for each[i] and allow garbage-collected erasure blocks to be used in bins wherethe erasure blocks are most needed, thereby allowing controller 12 toallocate over-provisioning to the bins that need more erasure blocks. Inother examples, controller 12 may modify the over-provisioningassociated with each bin [i] by adjusting the largest number of EB[i]that is allowed before bin [i] is selected for garbage collection.

In either example, controller 12 may use a multi-variate minimizationtechnique to determine a new D[i] or new EB[i], depending on methodchosen above. Or, controller 12 may determine the D[i] that are afunction of T[i] that will minimize equation (4). In addition,perturbation, adaptive processing, or similar techniques, may be used todetermine optimal choices to minimize equation (4).

The variable WriteFrequency(T) of equation (4) may be estimated asfollows:WriteFrequency(T)=(NPPEB−D(T))·f(T,D(T))·T  (6)where NPPEB is the number of pages per erasure block. As seen fromequations (4)-(6), the total amount of wear of an erasure block, i.e.,the value of TotalWear, is a function of SSD parameters (e.g., number ofpages per erasure block, etc.) and the over-provisioning for each T. Itshould be noted that the over-provisioning for each T can alternativelybe transformed into a garbage collection deadness threshold. Forexample, controller 12 may allocate a certain amount ofover-provisioning for a particular temperature, and then garbage collectthe erasure block that is “most dead” at that particular temperaturewhen a new erasure block is needed to be available at that particulartemperature.

As seen from the above, controller 12 may determine theover-provisioning across the set of all T values by an empiricalmulti-variate analysis technique (e.g., perturbation, matrix inversion,or the like), or by analytical methods, that adjust theover-provisioning to minimize wear with the additional constraint thatthe sum of the over-provisioning for each T at any point in time cannotexceed the amount of over-provisioned space in the entire SSD. In someexamples, the over-provisioning may be determined for an expectedworkload. In other examples, the over-provisioning may be adaptivelyadjusted during operation based on an estimated probability densityfunction of the workload.

Erasure blocks that are made available by garbage collection may be usedin a first-in first-out (FIFO) manner. However, in some examples, theerasure blocks may be allocated by controller 12 such that theirremaining wear capability is proportional to the estimated temperatureof the data for which the erasure block is being allocated. For example,an erasure block that has a high erasure count (indicating that theremaining wear capability of the erasure block is low) may be allocatedto store cold data. By its nature, cold data will not change frequentlyand, as such, may not induce significant wear in an erasure block. Incontrast, an erasure block that has a low erasure count (indicating thatthe remaining wear capability of the erasure block is high) may beallocated to store hot data. By its nature, hot data will changefrequently and, as such, may induce significant wear in an erasureblock. Thus, allocating cold data to highly worn erasure blocks andallocating hot data to erasure blocks with high remaining wearcapability takes advantage of properties of the data and the erasureblock.

Controller 12 may store the temperature of the data as metadata alongwith user data in erasure blocks 24 of non-volatile memory 20. Forinstance, controller 12 may populate a metadata table in each erasureblock that describes the state of each erasure block, including, forexample, the data storage area of the SSD to which the erasure blockbelongs and the estimated temperature of that data storage area at thetime the erasure block began filling with data. In other examples,controller 12 may store the temperature as metadata elsewhere. Forinstance, the metadata for the temperature of each erasure block may bestored in a system-wide table in the memory of controller 12 of SSD 10.Controller 12 may store the metadata in non-volatile memory and berecoverable. However, it is not necessary that the metadata berecoverable and, as such, the metadata may be stored in volatile memory,e.g., volatile memory 14.

In addition, controller 12 may store in a metadata table a time estimateof when the erasure block started to be filled again with data. Asmentioned above, controller 12 may use this time estimate to estimatethe heat of the data in the erasure block.

Controller 12 may store additional data as metadata. For example,controller 12 may store the physical location in an erasure block atwhich point new data written by the host was appended togarbage-collected data. That is, each time controller 12 stores data inone of the data storage areas of the SSD, controller 12 may keep trackof the location in the erasure block before which garbage-collected datawas written and after which new host data was written. Controller 12 maythen be able to identify the two different portions that have data attwo different temperatures than when the data was written into theerasure block.

Controller 12 may also store additional data, such as the approximateheat of the garbage-collected data at the time it was copied into theerasure block. During garbage collection, data copied into an erasureblock may have been copied from multiple erasure blocks. The data in thesource erasure blocks, however, may have had different temperatures. Inaccordance with techniques of this disclosure, during garbagecollection, controller 12 may assign one temperature estimate for alldata being copied into a single erasure block. That is, controller 12may assign one temperature estimate to garbage-collected data copiedinto a single erasure block from multiple erasure blocks. Controller 12may determine a single temperature estimate using numerous techniques.

As examples, controller 12 may use the minimum, maximum, or weightedaverage of the mappable units' estimated heat of the data when it waslast copied. During garbage collection, controller 12 may use thisadditional data to make a different decision as to where to copy thepreviously garbage-collected data than it makes for the data that wasnot previously garbage-collected, if the temperatures are sufficientlydifferent. For example, controller 12 may use this additionalinformation to separate out host data, which controller 12 may assignone temperature, from garbage-collected data, which controller 12 mayassign another temperature. In this manner, controller 12 mayeffectively segregate data.

As mentioned above, controller 12 may store data segregated intodifferent temperatures into different storage areas, e.g., erasureblocks. Controller 12 may use numerous temperature-segregating heatallocation strategies to store data segregated into differenttemperatures into different storage areas. For instance, controller 12may maintain a number Z erasure blocks for writing at any time, witheach of the Z erasure blocks assigned a different temperature of datathat may be written to the block. Of course, multiple erasure blocks maybe assigned the same temperature estimate to accommodate larger sets ofdata.

In other examples, controller 12 may assign data one of three differenttemperature estimates and then segregate data based on those threetemperatures into one of three storage areas. For instance, staticwear-leveled data may be “cold,” garbage-collected data may be “warm,”and host written data may be “hot.” Each of the three storage areas maybe a plurality of erasure blocks. In this manner, cold data may writteninto a first storage area, warm data may be written into a secondstorage area, and hot data may be written into a third storage area.Using the techniques described earlier in this disclosure, the firststorage area may have less over-provisioning than the second storagearea, and the second storage area may have less over-provisioning thanthe third storage area.

In another example, controller 12 may assign data one of one hundreddifferent temperature estimates and segregate data based on those onehundred temperature estimates into one of one hundred different storageareas. In such an implementation, controller 12 may use equation (1)above to estimate the heat of data written by the host. In this manner,controller 12 may assign all data to one of the one hundred differentstorage areas and then store the data in one of the data storage areasbased on the temperature categorization.

In another example, controller 12 may assign data one of two differenttemperature estimates and segregate data based on those two temperatureestimates into one of two different storage areas. In such animplementation, hot data may be written to the erasure blocks that havethe smallest erasure counts and therefore the most wear capabilityremaining, and cold data may be copied into erasure blocks whose wearcapability is at a threshold chosen so that the number of writesexpected for the cold data for the remainder of the SSD lifetime,compared to the number of hot-data writes expected for the remainder ofthe SSD lifetime equals the cumulative wear capability of the erasureblocks with as much or more wear than this threshold, compared to thecumulative wear capability of the erasure blocks with less wear thanthis threshold. In one example, the two different temperature estimatesmay be “hot” and “cold.” In such an implementation, controller 12 mayassign all data written by the host as “hot” data and all other data as“cold” data. Controller 12 may determine the threshold by maintaining aninfinite-impulse response (IIR)-filtered estimate of the amount of hotand cold data “seen” by controller 12.

By way of a simplified, specific example, assume that controller 12determines that 25% of the data being written is “hot” (i.e., from thehost), and 75% of the date being written is cold (i.e., copied by thegarbage collection process). Further assume that there are 1000 erasureblocks containing about 800 erasure blocks' of host data, of which thefollowing erasure capability exists:

TABLE 1 Erasures Total EB- No. of EB- Erasure Remaining EBs at thiserasures at erasures at or above Count in EB wear level this wear levelthis wear level Life-400 400 50 20000 100%  Life-350 350 50 17500 89%Life-300 300 75 22500 79% Life-250 250 100 25000 67% Life-200 200 12525000 53% Life-150 150 250 37500 40% Life-100 100 350 35000 19% Total1000 182500Table 1 illustrates, per erasure count wear level (column 1 from left),the following: the number of erasures remaining in an erasure block (EB)(column 2 from left); the number of erasure blocks in a wear level(column 3 from left); the total number of erasure block erasures at aparticular wear level (column 4 from left), and the number of erasureblock erasures at or above a particular wear level (column 5 from left).Using the techniques of this disclosure, controller 12 may write colddata to erasure blocks at an erasure count of Life-300 erases, or more,and write hot data to erasure blocks at an erasure count of Life-300erases, or less. Such an implementation may result in wearout of thelesser-worn erasures blocks by hot data at about the same time that thecold data wears out the already-more-worn erasure blocks. Controller12's selection of an erasure block for garbage collection at this timemay depend on whether controller 12 is searching for an erasure block towrite hot or cold data in to. If searching for an erasure block to writehot data in to, controller 12 searches for the most-dead erasure blockin the set of erasure blocks that is at-most worn to the Life-300 level.If searching for an erasure block to write cold data into, controller 12searches for the most-dead EB in the set of erasure blocks that isat-least worn to the (Life-300) level.

In another example data segregating technique, controller 12 may chooseto write data to an erasure block based on the likelihood of future hostwriting and the remaining wear capability of an erasure block. In suchan example, system 30 may use an erasure-block mapping scheme, ratherthan the page-mapping scheme described above.

Assume that system 30 defines time “t” as the number of erasure blockallocations that occurred prior to the last allocated erasure block.Further assume that system 30 maintains a “free list” containing anumber of erasure blocks that are available for allocation, and that asan erasure block has the last of its contained data invalidated (i.e.,the last of its contents are re-written into another erasure block, orare unallocated by a “trim” command, for example), the erasure block iserased and is returned to the “free list.” Controller 12 may sort theprobability density function (pdf) of all mappable host address sets bylikelihood of selection for writing by the host, i.e., the temperatureof the data.

FIGS. 13 and 14 are example graphs depicting the estimated probabilitydensity function of future host writes. FIG. 13 shows an example of anestimated, un-sorted pdf of the frequency of writing each of 100 LBAbands ( 1/100ths of the LBA range for an SSD). FIG. 14 shows the samedata as FIG. 13, but is sorted by temperature rather than by LBA. The100 LBA bands from FIG. 13 were sorted by heat to determine theestimated pdf in FIG. 14. In FIG. 14, the 100 LBAs are divided into fourequal-area sets 70A-70D, or heat-quartiles. In other words, each set70A-70D will each receive 25% of the writes, on average.

FIG. 15 is an example graph depicting the remaining wear capability of100 erasure blocks, sorted by wear rather than by erasure block number.The wear capability may be measured, for example, by the remainingerasure counts according to the manufacturer's specification. In FIG.15, the 100 erasure blocks are divided into four equal-area sets72A-72D, or wear-quartiles. Using the techniques of this disclosure, ifa host writes data addressed to an LBA in a particular heat-quartile,controller 12 may assign that data to an erasure block in a matchedwear-quartile. For example, controller 12 may match the integrationlimits in FIG. 14 to result in the same integral limits in FIG. 15,controller 12 may select an appropriate wear level for the next erasureblock to be allocated, which may allow controller 12 to avoid having toperform a predictably non-zero number of static wear-leveling copies ata later time. Although a smaller number of equal-area regions may reducethe number of times that random variation leads to a block with aparticular wear capability being deemed to have been sub-optimallymatched up to a new host block of a particular heat, a larger number ofallocation regions may improve the match up between the heat of data andthe wear capability of an erasure block.

FIG. 16 is a flow diagram illustrating an example method for segregatingdata in a non-volatile memory, in accordance with this disclosure.Controller 12 of FIG. 12, for example, provides two or more data storageareas in non-volatile memory (100). For instance, controller 12 mayprovide first data storage area 60 and second data storage area 62 innon-volatile memory 20 of FIG. 12. Then, controller 12 provides a firstamount of over-provisioning for a first of the two or more data storageareas and a second amount of over-provisioning for a second of the twoor more two data storage areas (105). Referring again to FIG. 12,controller 12 may provide first data storage area 60 with a number oferasure blocks 24 for over-provisioning (i.e., a number of erasureblocks greater than the number of erasure blocks advertised to a host,for example) and provides second data storage area 62 with a differentnumber of erasure blocks 24 for over-provisioning. Controller 12categorizes data based on a characteristic of the data (110). Forexample, controller 12 may categorize data based on a frequency of writeaccess of the data, i.e., the temperature of the data. Then, controller12 may store the data in one of the two data storage areas based on thecategorization (115). In one example, the data may be data written by ahost and/or data that is copied from at least a first erasure block toat least a second erasure block.

In some examples, the first amount of over-provisioning is greater thanthe second amount of over-provisioning, and storing the data in one ofthe two or more data storage areas based on the categorization includesstoring data categorized as having a higher frequency of write access inthe first of the two or more data storage areas. In one example,controller 12 may store the categorization of the data in at least oneof non-volatile memory 20 and a memory in controller 12.

In some examples, controller 12 may store the data in one of the datastorage areas based on the categorization by storing the data copiedfrom at least a first erasure block to at least a second erasure block,storing the data written by the host at a location adjacent to thecopied data, and then storing the location in at least one ofnon-volatile memory 20 and a memory in controller 12.

In one example, controller 12 may store, in at least one of non-volatilememory 20 and controller 12, e.g., memory within controller 12, thecategorization of the data that is copied from at least a first erasureblock to at least a second erasure block at a time when the data iscopied to the at least a second erasure block.

The techniques described in this disclosure may be implemented, at leastin part, in hardware, software, firmware or any combination thereof. Forexample, various aspects of the described techniques may be implementedwithin one or more controllers, one or more processors, including one ormore microprocessors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs), or any other equivalent integrated or discrete logic circuitry,as well as any combinations of such components. The term “controller,”“processor,” or “processing circuitry” may generally refer to any of theforegoing logic circuitry, alone or in combination with other logiccircuitry, or any other equivalent circuitry. A control unit comprisinghardware may also perform one or more of the techniques of thisdisclosure.

Such hardware, software, and firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

The techniques described in this disclosure may also be embodied orencoded in a computer-readable medium, such as a computer-readablestorage medium, containing instructions. Instructions embedded orencoded in a computer-readable medium may cause a programmableprocessor, or other processor, to perform the method, e.g., when theinstructions are executed. Computer readable storage media may includerandom access memory (RAM), read only memory (ROM), programmable readonly memory (PROM), erasable programmable read only memory (EPROM),electronically erasable programmable read only memory (EEPROM), flashmemory, a hard disk, a CD-ROM, a floppy disk, a cassette, magneticmedia, optical media, or other computer readable media.

The implementations described above and other implementations are withinthe scope of the following claims.

The invention claimed is:
 1. A method comprising: providing at least twodata storage areas in a memory; providing a first amount ofover-provisioning for a first of the at least two data storage areas anda second amount of over-provisioning for a second of the at least twodata storage areas; categorizing data being written by a host as havinga first designation; categorizing valid data being collected via agarbage collection operation as having a second designation; assigning asequence number to the data being written by the host; updating thesequence number based on the categorization; storing the data in eitherthe first or the second of the at least two data storage areas based onthe categorization and the sequence number; and selecting, for garbagecollection, erasure blocks containing cold data but having a highervalid data density before selecting erasure blocks containing hot databut having a lower valid data density.
 2. The method of claim 1, furthercomprising categorizing the data based on a frequency of write access ofthe data.
 3. The method of claim 2, wherein the first amount ofover-provisioning is greater than the second amount ofover-provisioning, and wherein storing the data in one of the at leasttwo data storage areas based on the categorization comprises: storingdata categorized as having a higher frequency of write access in thefirst of the at least two data storage areas.
 4. The method of claim 2,further comprising: storing the categorization of the data in at leastone of the memory and a controller.
 5. The method of claim 2, whereinthe data is at least one of data written by a host and data that iscopied from at least a first garbage collection unit to at least asecond garbage collection unit.
 6. The method of claim 5, wherein eachof the first garbage collection unit and the second garbage collectionunit comprise at least one erasure block.
 7. The method of claim 5,wherein storing the data in one of the data storage areas based on thecategorization comprises storing the data copied from at least the firsterasure block to at least the second erasure block and storing the datawritten by the host at a location adjacent to the copied data, andwherein the method further comprises: storing the location in at leastone of a non-volatile memory and a memory in a controller.
 8. The methodof claim 5, further comprising: storing, in at least one of the memoryand a memory in a controller, the categorization of the data that iscopied from at least the first erasure block to at least the seconderasure block at a time when the data is copied to at least the seconderasure block.
 9. The method of claim 1, further comprising: comparing acurrent sequence number assigned to the data being written by the hostto a sequence number previously assigned to the valid data beingcollected via the garbage collection operation; determining a differencevalue between the current sequence number and the sequence numberpreviously assigned to the valid data being collected via the garbagecollection operation; and categorizing the valid data being collectedvia the garbage collection operation as having the second designationwhen the determined difference value exceeds a threshold value.
 10. Themethod of claim 1, wherein categorizing data being written by a host ashaving a first designation comprises: comparing a current sequencenumber assigned to the data being written by the host to a sequencenumber previously assigned to previously written host data; determininga difference value between the current sequence number and the sequencenumber previously assigned to the previously written host data; andcategorizing the data being written by the host as having a firstdesignation based on the difference value.
 11. The method of claim 1,wherein the memory is non-volatile memory.
 12. The method of claim 1,wherein selecting, for garbage collection, erasure blocks having ahigher valid data density before selecting erasure blocks having a lowervalid data density comprises maintaining or increasing the first amountof over-provisioning.
 13. The method of claim 1, wherein selecting, forgarbage collection, erasure blocks having a higher valid data densitybefore selecting erasure blocks having a lower valid data densitycomprises selecting in response to the categorization of the data. 14.The method of claim 1, wherein the erasure blocks with the higher validdata density have a lower write frequency than the erasure blocks withthe lower valid data density.
 15. The method of claim 1, wherein thefirst designation includes data that is categorized as having a higherfrequency of write access and the second designation includes data thatis categorized as having a lower frequency of write access.
 16. A systemcomprising; a memory; and at least one controller configured to: provideat least two data storage areas in the memory; provide a first amount ofover-provisioning for a first of the at least two data storage areas anda second amount of over-provisioning for a second of the at least twodata storage areas; categorize data being written by a host as having afirst designation; categorize valid data being collected via a garbagecollection operation as having a second designation; assign a sequencenumber to the data being written by the host; updating the sequencenumber based on the categorization; store the data in either the firstor the second of the at least two data storage areas based on thecategorization and the sequence number; and select, for garbagecollection, erasure blocks containing cold data but having a highervalid data density before selecting erasure blocks containing hot databut having a lower valid data density.
 17. The system of claim 16,wherein the at least one controller is configured to: categorize thedata based on a frequency of write access of the data.
 18. The system ofclaim 17, wherein the at least one controller is further configured to:compare a current sequence number assigned to the data being written bythe host to a sequence number previously assigned to the valid databeing collected via the garbage collection operation; determine adifference value between the current sequence number and the sequencenumber previously assigned to the valid data being collected via thegarbage collection operation; and categorize the valid data beingcollected via the garbage collection operation as having the seconddesignation when the determined difference value exceeds a thresholdvalue.
 19. The system of claim 18, wherein the at least one controlleris further configured to update the sequence number by decrementing thesequence number when the determined difference value exceeds thethreshold value.
 20. The system of claim 17, wherein the at least onecontroller configured to categorize data being written by a host ashaving a first designation is further configured to: compare a currentsequence number assigned to the data being written by the host to asequence number previously assigned to previously written host data;determine a difference value between the current sequence number and thesequence number previously assigned to the previously written host data;and categorize the data being written by the host as having a firstdesignation based on the difference value.
 21. The system of claim 17,wherein the first amount of over-provisioning is greater than the secondamount of over-provisioning, and wherein the at least one controller,configured to store the data in one of the at least two data storageareas based on the categorization, is configured to: store datacategorized as having a higher frequency of write access in the first ofthe at least two data storage areas.
 22. The system of claim 17, whereinthe at least one controller is further configured to: store thecategorized data in at least one of the memory and the controller. 23.The system of claim 17, wherein the data is at least one of data writtenby a host and data that is copied from at least a first garbagecollection unit to at least a second garbage collection unit.
 24. Thesystem of claim 23, wherein each of the first garbage collection unitand the second garbage collection unit comprise at least one erasureblock.
 25. The system of claim 23, wherein the at least one controller,configured to store the data in one of the data storage areas based onthe categorization, is configured to store the data copied from at leasta first erasure block to at least a second erasure block and store thedata written by the host at a location adjacent to the copied data, andwherein the at least one controller is further configured to: store thelocation in at least one of the memory and the controller.
 26. Thesystem of claim 23, wherein the at least one controller is furtherconfigured to: store, in at least one of the memory and a memory in thecontroller, the categorization of the data that is copied from at leastthe first erasure block to at least the second erasure block at a timewhen the data is copied to the at least the second erasure block. 27.The system of claim 16, wherein the memory is non-volatile memory. 28.The system of claim 16, wherein the at least one controller is furtherconfigured to update the sequence number by incrementing the sequencenumber in response to a host write.
 29. The system of claim 28, whereinthe at least one controller is further configured to update the sequencenumber by decrementing the sequence number in response to a garbagecollection operation.
 30. A non-transitory computer-readable storagemedium comprising instructions encoded on the computer-readable mediumthat, upon execution, cause a controller within a device to: provide atleast two data storage areas in a memory; provide a first amount ofover-provisioning for a first of the at least two data storage areas anda second amount of over-provisioning for a second of the at least twodata storage areas; categorize data being written by a host as having afirst designation; categorize valid data being collected via a garbagecollection operation as having a second designation; assign a sequencenumber to the data being written by the host; update the sequence numberbased on the categorization; store the data in either the first or thesecond of the at least two data storage areas based on thecategorization and the sequence number; and select, for garbagecollection, erasure blocks containing cold data but having a highervalid data density before selecting erasure blocks containing hot databut having a lower valid data density.
 31. The non-transitorycomputer-readable storage medium of claim 30, wherein the instructionscause the controller to: categorize the data based on a frequency ofwrite access of the data.
 32. The non-transitory computer-readablestorage medium of claim 30, further comprising instructions that, uponexecution, cause the controller to: compare a current sequence numberassigned to the data being written by the host to a sequence numberpreviously assigned to valid data being collected via the garbagecollection operation; determine a difference value between the currentsequence number and the sequence number previously assigned to the validdata being collected via the garbage collection operation; andcategorize the valid data being collected via the garbage collectionoperation as having the second designation when the determineddifference value exceeds a threshold value.
 33. The non-transitorycomputer-readable storage medium of claim 30, wherein the instructionsthat cause the controller to categorize data being written by a host ashaving a first designation further comprise instructions that, uponexecution, cause the controller to: compare a current sequence numberassigned to the data being written by the host to a sequence numberpreviously assigned to previously written host data; determine adifference value between the current sequence number and the sequencenumber previously assigned to the previously written host data; andcategorize the data being written by the host as having a firstdesignation based on the difference value.
 34. The non-transitorycomputer-readable storage medium of claim 30, wherein the first amountof over-provisioning is greater than the second amount ofover-provisioning, and wherein the instructions that cause thecontroller to store the data in one of the at least two data storageareas based on the categorization comprise instructions that, uponexecution, cause the controller to: store data categorized as having ahigher frequency of write access in the first of the at least two datastorage areas.
 35. The non-transitory computer-readable storage mediumof claim 30, further comprising instructions that, upon execution, causethe controller to: store the categorization of the data in at least oneof the memory and the controller.
 36. The non-transitorycomputer-readable storage medium of claim 30, wherein the data is atleast one of data written by a host and data that is copied from atleast a first garbage collection unit to at least a second garbagecollection unit.
 37. The non-transitory computer-readable storage mediumof claim 36, wherein each of the first garbage collection unit and thesecond garbage collection unit comprise at least one erasure block. 38.The non-transitory computer-readable storage medium of claim 36, whereinthe instructions that cause the controller to store the data in one ofthe data storage areas based on the categorization comprise instructionsthat, upon execution, cause the controller to store the data copied fromat least the first erasure block to at least the second erasure blockand storing the data written by the host at a location adjacent to thecopied data, and wherein the computer-readable storage medium furthercomprises instructions that, upon execution, cause the controller to:store the location in at least one of the memory and the controller. 39.The non-transitory computer-readable storage medium of claim 36, furthercomprising instructions that, upon execution, cause the controller to:store, in at least one of the memory and the controller, thecategorization of the data that is copied from at least the firsterasure block to at least the second erasure block at a time when thedata is copied to the at least the second erasure block.
 40. Thenon-transitory computer-readable storage medium of claim 30, wherein thememory is non-volatile memory.